fix: skip tag step when no bump PR was opened

When the dispatched version is already on develop, bump-version.sh is
a no-op and peter-evans sets pull-request-operation=none with
pull-request-head-sha pointing at develop's existing HEAD. The tag
step then tried to create refs/tags/vX.Y.Z at that already-tagged
commit and failed with 422 "Reference already exists". Gate the tag
step on a PR actually being created/updated so no-op dispatches
exit cleanly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: leifdreizler

.github/workflows/bump_version.yml

@@ -37,6 +37,7 @@ jobs:
           sign-commits: true
 
       - name: Tag release on bump branch
+        if: steps.cpr.outputs.pull-request-operation != 'none'
         env:
           GH_TOKEN: ${{ steps.token.outputs.token }}
           SHA: ${{ steps.cpr.outputs.pull-request-head-sha }}